Automatic switchover system for radio transmitters



April`9, 1963 R. c:` wlLsoN ETAL AUTOMATIC SWITCHOVER SYSTEM FOR RADIOTRANSMITTERS original Filed Dec. 1o, 195e April 9, 1963 R. c. wlLsoNETAL 3,085,245

AUTOMATIC SWITCHOVER SYSTEM FOR RADIO TRANSMITTERS Original Filed Dec.l0, 1956 7 Sheets-Sheet 2 ATTORNEYS April 9, 1963 R. C. WILSON ETALAUTOMATIC SWITCHOVER SYSTEM FOR RADIO TRANSMITTERS Original Filed Dec.10, 1956 7 Sheets-Sheet if OSCILLATOR CONTROL CIRCUIT-8O l f/o/ esel/82A \l auFFER AMPLIFIER PIIAsE 'gg' L AMPLIFIER L STAGE T' INVERTERAMPLIFIER I cILLAmR x62 fag) my /04 I ,1,179 J f-- -I T 24o- FRoM 70 nam ,l l SIGNAL MONITOR 90 [74 I GOONFROI AMPLIFIER guus"- CG/ AND PHASELI. BRE-92 7a l oscILLo l INVERTER A/I;;I.IFIER I" m l wf-2;); F L- [7]/7-V cIRcuITs'Is TONE DETECTOR-74 l /84 BOOMFROM o-L- 8.P.F. 0.6. J/592/ ,AMPLIFIER SOON REcTIFIER AMPLIFIER IT.

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ATTO RN EYS April 9, 1963 R. c. WILSON ETAL AUTOMATIC SWITCHOVER SYSTEMFOR RADIO TRANSMITTERS 7 Sheets-Sheet 4 Original Filed Deo. lO, 1956 n.IruPk-Imrh O...

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ATTO RIEYS April 9, 1963 R. c. WILSON ETAL AUTOMATIC SWITCHOVER SYSTEMFOR RADIO TRANSMITTERS 7 sheets-sheet 5 Original Filed Deo. l0, 1956 OxON-.Nh

ATTORNEYS April 9, 1963 Original Filed Dec. 10, 1956 7 Sheets-Sheet 6END STATION-3H 334 I II-354 END STATION-SI2 550 I 33o.) 551 355, I 555)55/, I TRANsMT'R TRANSMTR glv I I IfATg'E'IOIm TRANsMT'R TRANsMT'RW98-000m- |79&000KC I uNIT I E UNIT |79a.eooI c I'TsaeooKcI Jam@ 1 Il I.'i I "E" 555 I TRA:Bl- R f55I TRANSFER DETITIDR UNITAND 332 I 352 UNITAND DEI'ggIoR RIF. DETEc. 340 I I 560 R.E DETEc. I I I l sa 5 9 AuDIoRECEIVER AuDIo I FILTER REGE'VER L FILTER I 240m |772.000KC 538 I L 358|772.000KC 350m I I 3/47 3/5 .523) 524 `I ITRANsuIrR TRANSM'TR mgoIl-RCENTER sTATIoN-3Io I I I I79s24oI c I19a.24oxc UNIT 32o ST7) *I ll I=QLIE 522 52e I TONE TRANSFER I 1 l i /g) I UN" AND d Z\ l IT l DETECTORREDETEQ FILTERI FILTER FILTER RECEIVER I 24D- 3so- 6oornaooo Kc I I IDas) :a4- I E MIXER -I I V555# d5 Z3/Y. ANTENNA 372 I cI-IANGEovR I uNIT367 TRANSFER :I j j '974` UNIT AND f- AMPLIFIER MDDULATN DETEc QR 577370) I I 570 r BEF. 353] TRANsM'TR TRANsMTR. I" 24o- I77z.oooI c'II:IT-r2.oooxc DRF Y 7a-I: 360' REFERENCE 559 380) 75 Y I STATION 3 .576-3I3 IITD'R FA'LURE 'TTEETIEE I EEDUIPMENT I INDICATOR "I'rsaooxc IINvENToRs FIG. 7

vv/Im, @uw QEQIWAIIDIQ I'-n ROBERT c. wILsoN and BY ROBERT J PUCHATYATTORNEYS April 9, 1963 R. c. WILSON ETAL.

AUTOMATIC SWITCHOVER SYSTEM FOR RADIO TRANSMITTERS 7 Sheets-Sheet '7Original Filed Dec. 10, 1956 United States Patent O 3,085,245 AUTOMATICSWITCHOVER SYSTEM FOR RADIO TRANSMITTERS Robert C. Wilson, Rockford,Ill., and Robert J. Puchaty Tulsa, Okla., assignors to SeismographService Corporation, Tulsa, Okla., a corporation of Delaware Originalapplication Dec. 10, 1956, Ser. No. 627,398, now Patent No. 3,019,435,dated Jan. 30, 1962. Divided and this application Mar. 20, 1959, Ser.No. 800,845

7 Claims. (Cl. 343--227) The present invention relates to radio positioninding systems and more particularly to improvements in radio positionfinding systems employing phase comparison in pairs of positionindication signals radiated from a plurality of spaced transmittingpoints to provide indications from which the position of a mobilereceiving point relative to the known positions of the transmittingpoints may be determined. More specifically, the present invention is adivision of copending application Serial No. 627,398, now U.S, PatentNo. 3,019,435, tiled December lO, 1956, and assigned to the sameassignee as the present invention.

In systems of the particular type referred to, the continuous wavesradiated from each pair of transmitters as received at the receivingpoint have a phase relationship which changes as a function of changingposition of Athe receiving point between the two transmitting points.More specifically, the waves radiated by each pair of transmitting unitsof the system are characterized by spaced isophase lines which arehyperbolic in contour about the transmitting points as foci. On a lineconnecting the pair of transmitters, these isophase lines are spacedapart a distance equal to one-half the mean wave length of the radiatedwaves and have diverging spacings at points on either side of this line.With this system arrangement, the position of a receiving point relativeto a pair of hyperbolic isophase lines may be determined by measuringthe phase relationship between continuous waves radiated from the pairof transmitters.

Since the point of location of the receiving point along the zoneseparating the two isophase lines is not indicated by such a phasemeasurement, it is desirable to employ at least three spacedtransmitters, different pairs of which function to provide a grid-likepattern of intersecting hyperbolic lines, in order to obtain absolutedetermination of the position of the receiving point. Systems of thecharacter described are exceedingly accurate insofar as the positionindications produced at the receiving point are concerned. To obtain thedesired indication accuracy, however, it is necessary to maintain phasesynchronization between the continuous waves radiated by the spacedtransmitters, or, alternatively, so to arrange the system that phaseshifts between the radiated waves .are compensated during the phasecomparing operation.

Phase synchronization of the waves radiated from the plurality oftransmitters presents an exceedingly difcult problem which has been thesubject of considerable development work. All solutions which have beenfound for this problem involve the use of relatively elaborate andsomewhat delicate instrumentation not well adapted for the continuity ofservice required in position determining system. To obviate thisproblem, systems of the continuous wave hyperbolic type have beenproposed (see Honore Paten-t No. 2,148,267, issued February 2l, l939),in which the phase shift problem is obviated by heterodyning t-hecarrier waves of each pair of transmitters at a tixed lin-k transmittingpoint, and modulating the difference frequency component of theheterodyned waves as a reference signal upon the carrier output of thelink ice transmitter for radiation to the receiving point, where thedifference frequency component is detected and phase compared with adifference frequency signal derived by directly -heterodyning .thetransmitted continuous waves a-t the receiving point. In this manner,phase shifts between the continuous waves radiated from the twotransmitters are completely compensated so that the measured phase angleis truly representative of the location of the receiving point between apair of isophase lines.

One system which utilizes the principles of the Honore patent but whichreduces the number of transmitting stations and frequency channelsrequired is disclosed in United States Patent No. 2,513,316 to James E.Hawkins. In the system disclosed and claimed in the latter patent, acommon link or reference transmitter is employed for modulating thereference signals developed upon a single space radiated carrier wave.Another arrangement which has been used to reduce both the amount ofequipment and the number of frequency channels is disclosed and claimedin United States Patent No. 2,513,317 to I ames E. Hawkins and Robert S.Finn. In the system disclosed in the latter patent two of the positionsignal transmitters are alternately rendered effective as link orreference signal transmitters under the control of signals alternatelyradiated from a third position signal transmitter. While these systemsprovide satisfactory operation, it has been necessary, until the presentinvention, to maintain operating personnel at each of the transmittingstations in order -to turn the equipment on or off whenever desired andto make certain that the equipment is functioning properly during use.Specifically, prior to the present invention, in order to turn on theequipment, it has been necessary to provide for voice communication,either by way of telephone lines or a radio transmitter between all ofthe transmitting stations, so that the operator at each station can beinstructed to turn on his equipment. Obviously, the use of such a largenumber of operators materially increases lthe cost of system operation.

The present invention concerns a system in which all of the transmittingstations may be turned on from a single control station and, hence, theremaining stations may be said to be remotely controlled. `In addition,the operation of the entire system may be monitored at a single point insuch manner that, in the event of operational failure, the source of thetrouble may be identified and maintenance personnel may immediately bedispatched to correct the difliculty. Thus, the present invention makesit possible to dispense with the use of operating personnel at all ofthe stations except the control station.

Accordingly, the principal object of the present invention is to reducethe number of persons required to maintain operation of a transmittingsystem of the hyperbolic, continuous wave type.

A further object of the present invention is to provide a transmittingsystem of the hyperbolic, continuous wave type which requires thecontinuous services of an operating crew at only one of the stations ofthe system.

It is also an object of the present invention to provide a transmittingsystem of the hyperbolic, continuous wave type in which the operation ofthe system is continuously monitored at a single station so thatoperational failures can be detected immediately.

Another object is to provide a transmitting system according to thepreceding object in. which the monitoring provides an indication of thesource of the operational failure so that the 'trouble can be correctedwithout delay.

A further object of the invention is to provide a transmitting system ofthe hyperbolic continuous wave type in which uninterrupted operation ofthe system is assured by the provision of standby transmitting apparatuswhich is rendered effective automatically in the event of equipmentfailure.

-It is likewise an object of the present invention to provide atransmitting system of the character indicated above having standbytransmitters at certain or at all of the transmitting stations andhaving equipment lfor automatically rendering each of these standbytransmitters operative in the event of failure of its associatedtransmitter.

It is `also an object of the present invention to provide a systemaccording to the preceding object in which the standby equipment isautomatically rendered operative periodically even in the absence ofequipment failure, thereby effecting longer operating life of thetransmitters of the system.

The invention, both as to its organization and method of oper-ation,together with further objects and advantages thereof, will best beunderstood by reference to the specification taken in conjunction withthe accompanying drawings in which:

FIG. l is a diagrammatic representation of a three- Ifoci transmittingsystem of the hyperbolic continuous wave type embodying the presentinvention;

FIG. 2 isa diagrammatic representation of the tone detector, the antennachangeover unit and the transfer unit and modulation detector employedat one of the end transmitting stations of the system illustrated inFIG. 1;

FIG. 3 is a diagrammatic representation of the osciliator controlcircuit and of the signal monitor unit employed at the centertransmitting station of the system illustrated in FIG. `1;

FIG. 4 diagrammatically illustrates the tone detector units and thecondition indicator employed at the center transmitting station of thesystem illustrated in FIG. 1;

FIG. 5 is a diagrammatic representation of the transfer unit employed atthe center transmitting station of the system illustrated in FIG. 1;

FIG. 6 is a diagrammatic representation of the equipment employed at thenetwork oiiice of the system illustrated in FIG. 1;

FIG. 7 is a diagrammatic representation similar to FIG. 1 butillustrating another embodiment of the present invention; and

FIG. 8 diagrammatically illustrates the failure indicator circuitemployed at the control station of the system shown in FIG. 7.

Referring now to the drawings, and, more particularly, to FIG. 1thereof, the invention is there illustrated as embodied in a three-focihyperbolic, continuous wave system for providing position information atany number of mobile receiving units (not shown) which may be carried byvessels or vehicles operating within the radius of transmission of aplurality of spaced apart transmitting units or stations 10, 11 and 12.The transmitting system illustrated in FIG. 1 is somewhat similar tothat disclosed and `claimed in the above identified U.S. Patent No.2,513,317 to Hawkins and Finn but, of course, additionally includes theremote control and monitoring equipment of the present invention. Thus,the transmitting units 11 and 12 are spaced -approximately equaldistances from the location of the transmitting station 10 and, as aresult, the transmitting station 10 may be referred to as a centertransmitting station, while the stations 11 and 12 may be called endtransmitting stations. To facilitate the ensuing description the endtransmitting station 11 will at times be referred to as the RED endstation and the receiving equipment designed to receive the wavesradiated from -this station will be termed a RED receiver while the endtransmitting station 12 will be referred to as a GREEN station andthereceiving equipment responding to the waves from station 12 will betermed a GREEN receiver. The stations 11 and 12Y are so positioned thatan imaginary base line interconnecting the points of location of units10 and 11 is angularly related to an imaginary base line interconnectingthe points of location of the units 10 and 12. -As is more fullydescribed hereinafter and as will be apparent from an understanding ofthe above identified Hawkins and Finn patent, the center transmittingunit 10 is equipped alternately to radiate a pair of position indicatingsignals so separated in frequency that they may be selectively receivedand distinguished. The end transmitting stations 11 and 12, on the otherhand, are equipped continuously to radiate position indicating signalswhich are alternately modulated with reference signals developed in amanner described more fully hereinafter.

Specifically, the center transmitting station 10 comprises a firsttransmitter or transmitting unit indicated generally a 14 and a secondtransmitter or transmitting unit indicated generally at 15. Only one ofthese transmitters is employed at a particular time, the remainingtransmitter being a standby or alternate unit which is automaticallyrendered effective by a transfer unit 16 in the event of transmitterfailure at the center Station. Since the transmitters 14 and 15 areidentical, a description of one of these units is deemed to besufficient. Thus, the transmitting unit 14 comprises a pair ofoscillators or signal generators 17 and 18 respectively developingsignals having frequencies of 1772.000 and 1798.000 kilocycles which aresupplied through an amplitude modulator 19 to a final amplifier 20. Theoscillators 17 and 18 are rendered alternately effective by alternatelysupplying plate voltage to the electron tubes of these oscillators. Thisaternate operation of the oscillators 17 and 18 may be effected by meansof a commutator 21 which is driven from a synchronous motor and geartrain 22, as fully described in the Hawkins and Finn patent referred toabove. Thus, as illustrated in FIG. l, the oscillators 17 and 18 arealternately keyed by alternately supplying anode current to theirelectron discharge tubes from the positive terminal 93 of a power supply94 through a commutating ring 21a which is shaft-connected as indicatedat 9S to be driven at constant speed by the synchronous motor and geartrain unit 22. More specifically, the positive terminal 93 of the anodecurrent source is connected to the conductive segment 2lb of thecommutating ring 21a, which segment spans slightly less than half thecircumference of the ring. The remainder of the ring is composed of aninsulating segment 21e. At diametrically opposed points on the peripheryof the ring are provided brushes 21d and 21e which are respectivelyconnected to positive conductors leading to the oscillators 17 and 18.Since the conductive segment 2lb of the ring represents slightly lessthan half the periphery surface, it will -be understood that a shortoff-signal period is provided between successive periods during whichthe oscillators 17 and 18 are alternately operated, thus preventingsimultaneous radiation of the two waves developed by these oscillators.The periodicity with which the two oscillators are alternately operatedis, of course, dependent upon the speed of rotation of the commutatorring, Preferably, this ring is driven at a speed of one revolution persecond, such that the oscillators 17 and 18 are each rendered operativeat one-half second intervals.

The oscillator 17 in the transmitter 14 and the corresponding oscillatorin the transmitter 15 may also be turned on by means of a manuallyoperated switch 96 which, when closed, bypasses the commutator andconnects the terminal 93 directly to the electron discharge tubes of theoscillators. Similarly, a manually operated switch 97 is provided tobypass the commutator and turn on the oscillator 18 of the transmitter14 and the corresponding oscillator of the transmitter 15. There is alsoprovided an on-oif switch 98 for controlling the delivery of A.C.current to the power supply 94 and to the remaining circuits at thecenter transmitting station 10. A switch 99 may be employed, if desired,to control the delivery of energizing voltage for the synchronous motorand gear train units of the transmitters 14 and 15. Thus, in order torender the center station effective to radiate continuously a 1772.000kilocycle signal during calibration or testing of the equipment, theswitches 96 and 9-8 are both closed. If, on the other hand, it isdesired to radiate continuously a 1798.000 kilocycle signal from thecenter station for test purposes or for calibration, the switches 97 and98 are both closed. To provide the normal operation of the centertransmitting station i in which 1772.000 and 1798.000 kilocycle signalsare alternately radiated in the manner described above, the switches 96and 97 are left in their open position and the switches 98 and 99 areclosed. The output signal from the final `amplifier of each of thetransmitters 14 and 15 is passed to the transfer unit 16 which selectsthe signal to be radiated and operates an antenna changeover unit Z3 topass the selected signal to an emitting antenna 24. The antennachangeover unit 23 samples the signal output of the unit 16 and suppliesa signal to the unit 16 which informs the latter to switch to thestandby transmitter in the event that signal output of the unit 16disappears. In view of the foregoing description, it will be recognizedthat, regardless of which of the transmitters 14 or 15 is in operation,the center transmitting Station 10 is effective alternately to radiateposition indicating signals having frequencies of 1772.000 and 1798.000kilocycles.

The equipment provided at the end station 11 includes a firsttransmitter 30 and a second transmitter 31, both of which are adapted todevelop signals having a frequency of 1798.600 kilocycles. In the eventof failure of one of the transmitters, the remaining transmitter, termeda standby transmitter, is adapted to be placed in operationautomatically by means of a transfer unit and modulation detector 32which functions to pass the signal developed by the operativetransmitter through an antenna changeover unit 33 to a radiating oremitting antenna 34. Transmitters 30 and 31 are identical and eachincludes an oscillator or signal generator indicated by the referencenumeral 35 in the transmitter 30. This oscillator develops signalshaving a frequency of 1798.600 kilocycles for passage through anamplitude modulator 36 and through a final amplifier 37 to the transferunit and modulation detector 32. The antenna changeover unit 33 alsosamples the output signal of the end transmitting Station 11 andsupplies this sample to the unit 32 so that in the absence oftransmission signal, the unit 32 -will automatically render the standbytransmitter effective.

For the purpose of `developing reference signals which are alternatelysupplied to the modulator circuits of the transmitters 30 and 31, theend station 11 also comprises a pair of receivers 38 and 39 which areidentical in construction, which are both tuned to a center frequency of1772.120 klocycles and which are both maintained in continuousoperation, so that in the event of failure of one of the receivers theother receiver will continue to effect proper operation of the equipmentat the end station 11. The receivers 38 and 39 are excited by signalspicked up by a common loop receiving antenna 40 which is so orientedthat signals radiated from the antenna 34 are nulled or eliminated,thereby minimizing receiver blocking problems which might otherwiseresult from the radiation of high power signals from. a point relativelyclose to the receiving antenna. The signals arriving from the centerstation 10 and from the end station 12, of course, initiate a responsein the antenna 40 and are passed to the signal input terminals of bothof the receivers 38 and 39. The output of the receiver 38 is passedthrough a band pass filter 41 tuned to a frequency of 240 cycles, whilethe output of the receiver 39 is passed through a similar band passfilter 42. As will be described more fully hereinafter, the receivers 38and 39 are adapted 4to heterodyne or beat the signals arriving from thecenter station 10 and from the end station 12 in order to develop the240 cycle difference frequency therebetween, which difference frequencyis, of course, passed through the filters 41 and 42`to the modulatorcircuits of the transmitters 30 and 31. The signals passed by thefilters 41 and 42 are also applied to a tone detector circuit 43 wherethey function in a manner to be described more fully hereinafter tocontrol the operation of the .transfer unit and modulation detector 32in order to permit the end transmitting station 11 to be turned on oroff from a remote point.

The equipment provided at the end station 12 is similar to that at theend station 11,` except, of course, for the frequency of operation ofcertain components of the transmitting and receiving circuits. Thus, theequipment at the end station 12 includes a first transmitter 50 and asecond ltransmitter 51 which are identical in construction and which areadapted to be rendered effective one at a time under the control of atransfer unit and modulation detector 52. The signals developed by theeffective transmitter are passed through an antenna changeover unit 53to a radiating antenna 54. The antenna changeover unit 53 samples theoutput of the transmitting station 12 and supplies a signal to the unit52 so that, in the absence of signal output from the station 12, thestandby transmitter is rendered effective automatically. The transmitter50, which, as indicated above, is identical to the transmitter S1,comprises an oscillator or signal generator 55 for developing signalshaving a frequency of 1772.240 kilocycles which are passed through anamplitude modulator circuit 56 and through a final amplifier 57 to thetransfer unit and modulation detector 52.

For the purpose of heterodyning the signals received from the centerstation 10 and from the end station 11 to develop the reference signalsreferred to above, the end station 12 is provided with a pair of wavesignal receivers 58 and 59 of identical construction, both center tunedto a frequency of 1798.300 kilocyclesand both maintained in continuousoperation. The input circuits of these receivers are each excited by thesignals developed by a loop receiving antenna 60 which, of course, is sooriented that the signals radiated from the antenna 54 are nulled inorder to eliminate or minimize the receiver blocking problems referredto above. The antenna 60 is, of course, excited by the signals arrivingfrom the center station 10 and from the end station 11 and these signalsare passed to the input of receivers 58 and 59'. The receivers 58 and 59are each adapted to heterodyne the signals received from the centerstation 10 and from the end station 11 and to develop the 600 cycledifference frequency therebetween. The latter difference frequency ispassed through 600 cycle band pass filters 61 and 62 to the modulationcircuits of `the transmitters 50 and 51, where 1t is amplitude modulatedupon the signal developed by the oscillator circuits of thesetransmitters. The signals passed by the filters 61 and 62 are alsoapplied to a tone detector unit 63 for controlling the operation of thetransfer unit and modulation detector 5,2 in order` to permit the endStation 12 to be turned on or off from a remote point as describedbelow.

.In view of the foregoing description, it will be recogmzed that eitherthe transmitter 30 or the transmitter 31 at the end station 11 isrendered effective to cause continuous radiation of a single carrierwave signal having a frequency of 1798.600 kilocycles, either thetransmitting unit 14 or the transmitting unit 1.5 at the center station1s operative to radiate alternately signals having frequencies of1772.000 and 1798.000 kilocycles, and either the transmitter 50 or thetransmitter 51 at the end station 12 1s effective to radiatecontinuously signals having a frequency of 1772.240 kilocycles. Duringthe interval when the center station 10 is radiating its 1772.000kilocycle signal, lthe receivers 38 and 39 at the end station 11 bothheterodyne the signal from the center station with the signalcontinuously radiated from the end station 12 to develop a 240 cyclebeat frequency which, as previously described, is passed through thefilters 41 and 42 to the modulator circuit of the effective transmitterat the end station 11. Thus, the signal radiated from the end station 11is alternately modulated with a 240 cycle reference signal. The loopreceiving antenna 40 cooperates with the receivers 38 and 39 to preventthe appearance of the signal radiated from the end station 11 at theoutput of the receivers 38 and 39. The modulated carrier wave radiatedby the end station 11 is, of course, accepted by the receivers 58 and 59at the end station 12, but the 240 cycle reference signal has no eifecton the operation of the end station 12 in view of the fact that it isrejected by the lters 61 and 62. This reference signal may, if desired,be reproduced at the end station 12 and used during installation of thesystem in yCalibrating to .adjust the frequencies of the transmitters 50and 51 for monitoring or for other similar purposes.

During the interval when the center transmitting station is effective toradiate signals having a frequency of 1798.000 kilocycles, the receivers58 and 59 at the end station 12 heterodyne the signal received from thecenter station with the signal continuously radiated from the endstation 11 in order to develop a 600 cycle beat frequency which ispassed through the lters 61 and 62 to the modulating circuit of theeffective transmitter at the end station 12. Thus, the signal radiatedby the end station 12 is alternately modulated with reference signalshaving a frequency of 600 cycles. The latter reference signal appears atthe output of the receivers 38 and 39 at the end station 11 and may beused for monitoring or calibration as described above, but it isprevented from affecting the operation o-f the end station 11 due to thefact that it is rejected by the filters 41 and 42.

'Ihe signals radiated from all three of the transmitting units 10, 11and 12 are received at a mobile receiving unit of the type described inthe above-identified Hawkins and Finn patent where the received signalsare employed to develop a pair of position indications representative ofthe location of the mobile receiving unit relative to the threetransmitting stations. Thus, as will'be apparent from. an understandingof the Hawkins and Finn patent, the reference signal modulated upon thecarrier wave radiated from the end station 11, the carrier wavecontinuously radiated from the end station 12 and the 1772.000 kilocyclewave radiated from the center station 10 are employed to develop a rstposition indicated representative of the location of the mobilereceiving unit relative to hyperbolic isophase lines having foci at thecenter station and at the end station 12. Similarly, the 600 cyclereference signal modulated upon the carrier wave radiated from the endstation 12, the carrier wave radiated from the end station 11 and thev1798.000 kilocycle wave radiated from the center sta-tion 10 are usedto provide a position indication representative of the location of themobile receiving unit relative to hyperbolic isophase lines having fociat the center station 10 and at the end station 11.

In addition to the two transmitting -units 14 and 15 previouslydescribed, the center station 10 also includes means for monitoring thesignals radiated from the transmitters to provide a continuous check onthe operation of the systems, to enable the operator to evaluate thequality of emission and to facilitate frequency adjustment so that thedesired audio beat frequencies can be maintained. Moreover, the centerstation includes apparatus for cooperating with the equipment at the endtransmitting stations to eifect remote control of the latter stations sotha-t the entire transmitting system may be turned on from the centerstation without requiring the presence of operators at the two endstations. In addition, a monitoring station indicated by the referencenumeral 70 may be employed either at the tra-nsmitting station 10 or ata location remote from all three of the transmitting stations, such asat the network oice, to monitor the system in order to ascertain whetherthe equipment is functioning properly and -to locate the source oftrouble in the event that failure occurs.

Specically, the center .transmitting station 10 comprises, in additionto the equipment previously described, a pair of receivers 71 and 72respectively tuned to frequencies of 1772.240 kilocycles and 1798.600kilocycles, both of which are excited by signals received at a looppickup antenna 73. The antenna 73 is, of course, oriented so that thesignals emitted from the end stations 11 and 12 are accepted and appliedacross the signal input terminals of both of the receivers 71 and 72while the signals emitted from the antenna 24 are nulled and eliminated.The receiver 71 is suiciently selective'to reject the 1798.600 kilocyclesignal arriving from the station 11 while accepting the 1772.240kilocycle signal arriving from the end station 12. The 600 cyclemodulation component of the signal received from the end station 12 isreproduced by the receiver 71 and is passed through a band pass lter 92-to a pair of tone detector circuits 74 and 75. The receiver 72, on theother hand, is suiciently selective to reject the signal received by theantenna 73 from the end station 12, but the signal received from the endstation 11 is accepted and the 240 cycle modulation component thereof isreproduced and passed through a 240 cycle band pass lter 76 to the tonedetectors 74 and 75. The tone detectors 74 and 75 are employed tocontrol the operation of a condition indicator circuit 77, whichindicates the absence of one or both of the signals from the filters 76and `92. The signals passed by the filters 76 and 92 are also appliedthrough one section 78 of an oscillator selector switch 79 to a signalmonitor 90 described more fully below.

To provide for initiation of the operation of the end stations 11 and 12from the center station 10, there is provided at the latter station anoscillator control circuit indicated generally at 80 for developingaudio frequency signals to be modulated upon the wave radiated from thecenter station. This oscillator control circuit includes a pair ofhighly stable audio Afrequency generators 81 and 82 for developingsignals having frequencies of 240 cycles and 600 cycles, respectively.The outputs of these two signal generators are respectively applied tospaced apart contacts 83a and 83b of section 83 of the switch 79. Theswitch 79 may be employed to apply the output of either the generator 81or the generator 82 through an amplifier 84 and through a modulationon-oif switch 85 to the modulator circuits of the transmitters 14 and15. In addition, the switch 85 applies the output of the amplilier 84 tothe signal monitor 90 where it is compared with the audio signal supliedthrough the switch section 78.

Essentially, the remote control equipment comprises means including theoscillator control circuit 80 for modulating the 1772.000 kilocyclesignal radiated from the center station 10 with a 240 cycle toneswitching signal, means at one of the end stations, for example, at thestation 11 for receiving the switching signal and, after a predeterminedtime delay, turning on the end station 11, after which the end station12 will be turned on automatically in response to the 1798.000 kilocyclesignals alternately radiated from the center station and to the 1798.600kilocycle signal emitted from the end station 11. The end station 12 mayalso be turned on by the modulation of a 600 cycle tone modulationsignal upon the 1798.000 kilocycle signal radiated from the centerstation 10. The circuits at the end station are so designed that thetone switching signals must be applied throughout the predetermineddelay on period, thus preventing the transmitting stations from beingturned on by spurious noises or the like. However, the time constants ofthese circuits are such that the normal switching rate taking place atthe center station and produced by the commutator 21 does not affect theoperation. By proper use of the facilities provided, the center stationoperator may turn on either of the end stations without turning on theother for purposes of testing, calibration or the like, or he may turnon both of the end stations in order to place the entire transmittingsystem in operation. The end stations may also be turned oifautomatically merely by rendering the center station inoperative,thereby to terminate the development of tone signals at the endstations. Even after the tone signal development has ceased, the endstations remain on for a predetermined delay off period somewhat inexcess of the delay on period for purposes developed more fully below.

Turning now to the oscillator control circuit 80 provided at the centerstation for developing the tone switching signals referred to above andreferring particularly to FIG. 3 of the drawings, it will be observedthat this circuit includes a pair `of audio oscillators 81 and 82. Theseoscillators are of the feedback type having a vibrating reed in thefeedback circuit for stabiilzing the frequencies of the generatedsignals. As indicated above, the oscillator 81 is designed and adjustedto develop an audio frequency signal of 240 cycles, while the oscillator82 develops an audio frequency signal of 600 cycles. To increase thestability of the oscillators and to immunize them from load changes,their outputs are applied through a conventional buffer amplifier 101 tosection 83 of the oscillator selector switch 79. Specifically, the 240cycle signal developed by the oscillator 81 appears between contact 83aof switch section 83 and ground, while the 600 cycle signal developed byoscillator 82 appears between contact 83b and ground. Thus, the movablearm or pole of switch section 83 may be selectively controlled to applyeither the 240 cycle or the 600 cycle frequency to the amplifier 84. Thelatter amplifier comprises an input amplifier stage 102 having itssignal input terminals connected to the movable arm of the switchsection 83 and a phase inverter circuit 103 for developing push-pullsignals for `application to an output push-pull amplifier 104 ofconventional construction. The signal output terminals of the push-pullamplifier 104 are connected through the modulation on-off switch 85 tothe modulator circuits `of both of the transmitters 14 and 15. Theswitch 85, when closed, is also effective to deliver the output signalsof the push-pull amplifier 104 to the signal monitor 90 described morefully below. In view of the foregoing descripiton, it will be recognizedthat by the selective manipulation of the oscillator selector switch 79the modulator circuits of the transmitters 14 and 15 may be suppliedeither with a 240 cycle tone switching signal or with a 600 cycle toneswitching signal.

Let it be assumed for the present that it is desired to turn on the endstation 11 While, at the same time, maintaining the end station 12 inits inoperative condition. Such a condition might be desirable forpurposes of monitoring the end station 11 or to facilitate frequencyadjustment and the like. To effect the desired result, the oscillator`selector switch 79 is so manipulated that the signal appearing betweencontact 83a of switch section 83 and ground is applied to the input yoflthe amplifier 84, the modulator on-off switch 85 is closed, ltheoscillator control switch 96 is closed and, of couurse, the main poweronoff switch 98 is closed while switches 97 and 99 remain open. In thismanner the center station is rendered effective continuously to radiatea 1772.000 kilocycle carrier` wave which is modulated by the 240 cyclesignal developed by the oscillator control circuit 80. This modulatedcarrier wave signal is, of course, rejected by both of the receivers 58and 59 at the end transmitting station 12 and, hence, has no effect onthe operation of the latter station. The 240 cycle audio switchingsignal is, however, reproduced by both of the receivers 38 and 39 at theend transmitting station 11 and is passed through filters 41 and 42 tothe tone detector circuit 43. As previously mentioned, both of thereceivers 38 and 39 are maintained in operation continuously so thateach will be conditioned to receive the signal from the center station.The two signals passed by the filters 41 and 42 are applied asillusstrated in FIG. 2 to separate sets of signal input terminals of alinear mixer circuit 106 in the tone detector 43. The linear mixercircuit functions to mix the two input signals without developing sumand difference frequencies and passes both of the 240 cycle signals toan amplifier circuit 107. The output of the latter amplifier is passedthrough a 240 cycle audio band pass filter 108 to a halfwave rectifiercircuit 109. The latter rectifier circuit clips off the positivehalf-cycles of the 240 cycle input signal and passes the negativehalf-cycles through a resistancecapacitance filter network to thecontrol grid of a D.C. yamplifier 110. The time constant of thisresistancecapacitance filter network, as described more fully below, issufficient -to maintain the negative signal on the grid of amplifier 110during a period corresponding to the normal -switching rate effected bythe commutators in the transmitters 14 and 15 at the center station. Thenegative signal applied to the control grid of the D.C. amplifier 110 issufiicient to cut off the plate current of this amplifier and, sincethis plate current flows through the operating coil 111 of a tonedetector relay 112, it will be apparent that whenever 240 cycle signalsare passed by the filter 108, the relay 112 is opened or de-energized.The audio filter 108 in the tone detector prevents spurious signals orinterference from opening the tone detector relay and effectivelyprevents opening of this relay except during periods when a 240 cycletone signal is being developed by receivers 38 and 39. The linear mixer106 in the input of the tone detector circuit allows the relay 112 to beopened when a signal is developed either by the receiver 38 or thereceiver 39. Thus, the tone detector 43 will respond to a 240 cyclesignal developed by either one of these receivers, even though the otherreceiver may be ineffective due to operational failure or the like. Inany event, prior to the development of the 240 cycle switching signal bythe receivers 33 and 39, the D.C. amplifier 110 is in a conducting stateand its plate current flow energizes the relay 112 to hold pole 112a inengagement with stationary contact 112b. As soon as a 240 cycle signalis developed by either of the receivers 38 and 39 as, for example,4byreproducing the 240 cycle tone switching signal modulated upon the1772.000 kilocycle wave being continuously radiated from the centerstation 10 under the described conditions, the relay 112 opens and pole112a drops into engagement with a normally closed contact 112e, therebyapplying voltage to a time `delay relay 113 which may be referred to asthe delay on relay. Specifically, when the relay 112 is opened, oneterminal 114 of an alternating: current source is connected through therelay 112, through a signal connector 115, through the normally closedcontact 113a of the time delay relay 113 to a time delay motor 116 orthe like in the relay 113. The other side of the `time delay motor 116is permanently connected through a conductor 117 to the second terminal118 of the A.C. source.

Thus, as soon as relay 112 opens, the time delay motor is energized andthe timing cycle of the delay on period begins to run. After apredetermined delay which, in accor-dance with a particular installationof the present invention, is set at 60 seconds, the operating coil 119of the relay 113 becomes energized to move the pole 113b of the relayinto engagement with normally open contact 113e, thereby applying theline voltage across the operating coil 120 of a latching relay 121. Thelatter relay is of the type having its contacts mechanically latchedboth in the open and closed positions and, hence, the relay contactsremain closed. The time delay relay 113 is of the instantaneous resettype, so that if the signal supplied -to the D.C. amplifier 110 isterminated during the delay on period for a period of time in excess -ofthe time constant of the RC network in the input circuit of thisamplifier, the relay 112 is closed and the relay 113 is reset so thatthe timing cycle must be re-initiated. The instantaneous reset featureof the relay 113 thus prevents short or intermittent spurious signalssuch as static ior the like from operating the latching relay 121. Whenthe coil 120 of the latching relay is energized, contacts 121a and 121bare closed to complete a circuit through a ratchet relay 122 to one ofthe Itransmitters 30 or 31 at the end station 11. The condition orposition of the ratchet relay will, of course, determine which one ofthe transmitters is turned on initially. Thus, for example, when theratchet relay is positioned as illustrated in FIG. 2, the power supplyfor the transmitter 31 is delivered through a bus conductor 123, throughthe now closed contact 121a, through a connector 124, through closedcontact 122a of the ratchet relay, through a signal connector 125,through a signal light 129 for indicating that transmitter 31 is on,through the operating coil 126 of an antenna changeover relay 127 andthrough a connector 128 to the plate and screen circuits of the electrondischarge tubes of the transmitter 31. Plate voltage is not `deliveredto the electron discharge tubes of the transmitter 30, in view of thefact that contact 122b of the ratchet relay 122 is open. The flow ofcurrent through the operating coil 126 of the antenna changeover relaycloses contact 127a and connects the output of the final amplifier ofthe transmitter 31 through a coaxial cable y130 to the antenna 34.

With the circuits of the transmitter 31 energized, the 240 cycle signalreproduced by either or both of the receivers 38 and 39 is passed to themodulator circuit of the transmitter 31, where it is amplitude modulatedupon the carrier wave radiated from the antenna 34. A portion of thesignal supplied to the antenna 34 is applied through a suitable couplingcircuit or sampling means toa modulation detector circuit indicatedgenerally at 138. This coupling circuit may comprise a small capacitorconnected to the inner conductor of the coaxial cable 130 for supplyingsignals through a connector 132 to the signal input circuit of adetector 131 at the input of circuit 138. The latter detector is ofconventional construction and functions to demodulate the input signaland to reproduce the 240 cycle modulation component which is passedthrough a transformer coupled amplifier 133 to a half-wave rectifier134. The rectifier 134 is similar to the rectifier 109 described aboveand functions to eliminate the positive half-cycles and to pass thenegative half-cycles of the 240 cycle signal to the control grid Vof aD.C. amplifier 135. The latter amplifier, like the amplifier 110described above, includes in its input circuit a resistance-capacitancenetwork having a time constant Vsufficient to maintain the negativevoltage during the normal switching rate of the transmitting system. Thenegative voltage supplied by the rectifier 135 has a magnitudesuliicient to cut olf the amplifier 134 whenever signals are supplied tothe cable 130. Thus, when the 240 cycle signal is modulated upon thewave radiated from end station 11, the amplifier 135 is cut off and theplate current flow to the operating coil 136 of a modulation detectorrelay 137 is terminated. Thus, before the latching relay 121 has beenenergized to turn on one of the transmitters at the end station 11 theamplifier 135 is in a conducting state, the relay 137 is actuated andits contact 137a is closed. As soon as a 240 cycle signal is reproducedat the end station 11 to operate the relay 112, current is passedthrough the connector 115, through the normally closed contact 137a,through the normally closed contact 140a of a time delay relay 140` andthrough the timing motor 141 of the time delay relay to the connector117. Thus, as soon as the tone detector relay 112 is opened incident tosignal reception at the end station 11, the cycle of the timing motor141 is initiated. The time delay effected by the relay 140 isconsiderably less than that provided by the delay on relay 113 and, inaccordance with a particular embodiment of the present invention, isadjusted to seconds. Thus, at the end of the 5 second delay period theoperating coil 142 of the relay 140 is energized to close the normallyopen contact 140b in order to apply line voltage across the operatingcoil 143 of the ratchet relay 122. Actuation of the coil 143 draws anoperating pawl 144 into engagement with the teeth of a ratchet wheel 145to advance the relay 122 one step. The ratchet 145 is fxedly mounted ona shaft indicated at 146 which also carries a pair of switch operatingcams 147 and 148 having cam teeth which are so spaced that the contacts122a and 122b will each be alternately opened and closed as the ratchetrelay is advanced. Specifically, when the operating coil 143 is actuatedto advance the ratchet relay from the position illustrated in FIG. 2,the contact 122a will be opened, due to the fact that movable arm 149will drop into one of the recessed regions on the periphery of the cam147. At the same time, contact 122b is engaged by arm 150, due to thefact that this arm is moved radially outward by engagement with atoothed region on the cam 148. When the contact 122a is opened, ofcourse, the plate voltage supply to the electron discharge tubes of thetransmitter 31 is interrupted and this transmitter is, of course,rendered ineffective. Closing of the contact 122b conditions the platesupply circuit for the transmitter 30 to be completed as soon as thelatching relay 121 is operated at the completion of the delay on periodeffected by the relay 113. Thus, at the start of each turn-on period atthe end transmitting station 11, the ratchet relay 122 is operated totransfer from one of the transmitters 30 or 31 to ythe othertransmitter. `In this manner, extended use of a signal transmitter overa long period of time is prevented and the operating life of each of thetransmitters 30 and 31 is lengthened.

Some time after the ratchet relay 122 has been operated in the mannerdescribed above, the timing cycle of the delay on relay 113 will becompleted to operate the latching relay 121 in the manner describedabove, whereupon a circuit will be completed from bus conductor 151through the now closed contact 12111, through a connector 152, throughthe now closed contact 122b of the ratchet relay 122, through a signallight 154 and through a signal connector 153 to the plate and screengrid circuits of the electron discharge tubes of the transmitter 30,thereby to place the latter transmitter in operation. The on conditionof the transmitter 30, of course, is indicated by illumination of thesignal light 154.

During calibration or for test purposes, the transmitters 30 and 31 maybe turned on manually at the end station by means of a switch 155 whicheffectively bypasses the time delay circuits and the various relaysdescribed above. Specifically, when the switch 155 is moved to adownward position, illustrated in FIG. 2, a direct connection isprovided between conductor 151 and conductor 153, thereby deliveringplate voltage to the tubes of the transmitter 30 to place the lattertransmitter in operation. During this time, of course, the transmitter31 is not operated due to the fact that the lower section of the switch155 is confronted by an open circuit condition. Under these conditions,the antenna changeover relay 127 is not energized and the output oftransmitter 30 is connected to the antenna circuit. When, however, theswitch 155 is moved to the upper position illustrated in FIG. 2, theconnection between conductor 151 and conductor 153 is broken and theconductors 123 and 125 are connected directly together to deliver platevoltage to the tubes of the transmitter 31. Under the latter conditions,the antenna changeover relay 127 is operated to transfer the antennacircuit connection from the output of the transmitter 30 to the outputof the transmitter 31.

To summarize briefly the turn-on procedure described above, it will beobserved that the presence of the 240 cycle tone switching signal at theoutput of either of the receivers 38 or 39 operates the tone detectorcircuit to close the relay 112. The latter operation starts the timedelay relays 113 and 140. At the end of a delay period of a few seconds,the relay is effective to operate the ratchet relay 122 and conditionthe operating circuits for the transmitters 30 or 31. Some time later,the delay on relay 113 is effective to energize the latching relay 121and complete the connection to one of the transmitters 30 or 31 throughthe ratchet relay 122. Thus, each time the end station 11 is turned on,the ratchet relay 122 and its associated circuitry are effective totransfer from one of the transmitters 30 or 31 to the other transmitter.When the latching relay 121 has been energized and one of thetransmitters 30 or 31 has been placed in operation, the antennachangeover relay 127 is automatically rendered effective to connect theantenna circuit to the energized transmitter.

When the station 11 has been turned on, the modulation detector circuit138 opens the relay 137 to break the circuit to the relay 140. The relay137 remains open as long as a modulated signal is present on the cable130 and, hence, the circuit at the end station will remain at thedescribed position. If, however, the signal on the cable 130 disappearsas, for example, by failure of the effective transmitter at the endstation 11, the negative voltage supplied to the control grid of theD.C. amplifier 135 disappears to render this tube conducting and toclose the modulation detector relay 137. At this time it will beremembered that a 240 cycle signal is still present at the output of thereceivers 38 and 39 and, hence, tone detector relay 112 remains open. Assoon as relay 137 is closed, the timing motor 141 of the time delay 140is energized. If, during the delay period provided by the relay 140, theequipment failure is corrected, that is, if the trouble occurred as aresult of momentary power failure or the like at the station 11, therelay 137 is immediately opened and the time delay relay 140 reverts toits original condition without having any effect on the operation of theratchet relay 122 or the other circuits illustrated in FIG. 2. If,however, the failure does not correct itself within the delay periodprovided by the relay 140, the relay operating coil 142 is energized toclose contact 140b, thereby to advance the ratchet relay 122. Thelatching relay 121 is, of course, closed at this time and, accordingly,the advancement of the ratchet relay irnmediately places the standbytransmitter in operation, thereby automatically energizing the standbytransmitter in the event that the effective transmiting equipment fails.

When it is desired to turn off the end transmitting station 11, theoperator at the center station may remove he 240 cycle modulation signalas, for example, by opening the modulation on-off switch 85. When thismodulation is removed, the output from the receivers 38 and 39 isinterrupted and the D.C. amplifier 110 is no longer excited by negativesignals from the rectifier 109. Therefore, the amplifier 110 begins toconduct and its plate current flow energizes the relay 112 to closecontact 112b in order to complete a connection through conductor 157 andthrough normally closed contact 158'a of a delay off time delay relay158' to the timing motor 159 of the latter relay. The delay off periodeffected by the relay 158 is considerably longer than the delay onperiod provided by the relay 113 and, in accordance with a particularinstallation of the present invention, this delay period was selected at90 seconds. During this delay off period the end transmitting station 11remains in operation so that if the absence of signal output atreceivers 38 and 39 is due to a momentary failure of theequipment eitherat the center transmitting station or at the end transmitting station11, the operation of the end transmitting station can be resumed as soonas the momentary failure is corrected. Thus, in the event that 240 cyclesignals are again produced by either of the receivers 38 or 39` duringthe 90 second delay off period, the relay 112 is again opened and thedelay off relay 158 reverts to its normal position, illustrated in FIG.2. The delay off circuit thus prevents the transmitting equipment at theend Station 11 from shutting down in case of momentary power failure atthe end station 11, thereby to provide a period of time sufficient forautomatic starting type generators to pick up the load and correct thepower failure. In addition, the delay off circuit holds the end station11 on the air for a period of time which is sufficient to allow theentire system to be placed into operation during a complete turn-onperiod as described below. At the end of the delay o period, theoperating coil 160 of the relay 158 is energized to close contact 15819and, hence, to energize the turn off operating coil 162 of the latchingrelay 12'1 via conductor 161. Energization of the coil 162 releases themechanical latch holding the mov able arms of the relay 121 in the downposition and draws these arms upwardly, thereby interrupting both of thecircuits supplying power to the transmitters 30 and 31. Thus, both ofthese transmitters are rendered inoperative and the end station 11 goesoff the air.

The equipment provided at the end transmitting station 12 to effect theautomatic transfer between the transmitters 50 and 51 and to provide theremote control operation is similar to the equipment just described atthe transmitting station 11. Thus, the tone detector 63 is identical tothe tone detector 43 just described except, of course, that the audiofilter in the detector 63 is tuned to pass signals having a frequency of600 cycles, while the audio filter 108, as indicated above, passes 240cycle signals. The frequencies of operation of the remaining circuits atthe end station 12 are changed in similar manner. Therefore, in theevent that it is desired to turn on the end station 12 for test purposesor for calibration while, at the same time, maintaining the end station11 in inoperative condition, the operator at the center station mayleave switches 96 and 99 open, close the main power on-off switch 98,close the manually operated switch 97, close the modulation on-offswitch and place the oscillator selector switch 79 in position todeliver 600 cycle signals from the oscillator control circut 80 throughthe amplifier 84 to the modulator circuits lof the transmitters 14 and15. `In this manner the center station 10 is rendered effectivecontinuously to radiate a 1798.000 kilocycle carrier wave amplitudemodulated with a 600 cycle switching signal. This signal is received bythe antenna 60 and is passed to the receivers 58 and 59 Where the 600cycle modulation. component is detected and passed through filters 61and 62 to the tone detector 63. The tone detector 63 responds to theinput signal by opening its tone detector relay in order to turn on thetransmitting equipment at the end station 12 in exactly the same manneras the turn-on lprocedure which takes place at the end transmittingstation 11 and which has been described previously. The end transmittingstation 12 is, of course, turned off by removing the 600 cycle toneswitching signal modulated upon the carrier wave radiated from thecenter station as, for example, by opening the modulation on-ofi switch85. In the absence of signal output at both of the receivers 58 and 59,the tone detector relay is closed and the transmitting station 12 isturned off in exactly the same manner as the turn-off procedure employedat the end station 11. The transfer unit and modulation detector 52 issupplied with a sample output from the antenna changeover unit 53 inorder to effect automatic transfer to the standby transmitter in theevent of operational failure of the main transmitter in the mannerpreviously described. Also, the unit 52 is effective to transfer fromone of the transmitters 50 or 51 to the other transmitter each time theend station equipment is turned on in order to instrument thetransmitter equipment-sharing feature previously described.

Assuming that the transmitting station 11 has been turned on in the samemanner described above by continuously radiating from the center station10 a 1772.000 kilocycle signal modulated with a 240 cycle tone detectorsignal, the 240 cycle signal modulated on the wave radiated from the endstation 11 is detected and is passed through the band pass filter 76 andthrough section 78 of the modulation on-of switch 79 to the signalmonitor in the manner illustrated in FIG. 3. Specifically, with bothsections of the switch 79 in their upper position, illustrated in FIG.3, the output of the band pass filter 76 is developed across a gaincontrol potentiometer 170. This potentiometer may be varied to controlthe amplitude of the input signals supplied to an amplifier and phaseinverter unit 171 which develops push-pull signals for eX- citing aconventional push-pull amplifier 172. The output of the push-pullamplier 172 is applied to one set of deflection plates of anoscilloscope 173, the other pair of deflection plates of which areexcited by the signal output of the push-pull ampliier 104 suppliedthrough switch 85. Thus, a lissajous pattern is produced on the screenof the oscilloscope in response to the two 240 cycle signalsrespectively applied to its horizontal and vertical plates. Aspreviously indicated, the 240 cycle signal developed by the oscillator81 is highly stable and, hence, serves as a frequency standard. Thus,the lissajous pattern may be observed by the operator at the centerstation to make certain that the transmitting equipment at the endstation 11 is functioning properly and also to facilitate calibrationand frequency adjustment of the transmitting equipment at the centerstation. The output of the pushpull ampliiier 172 may also be applied toa loudspeaker or the like in order to produce a continuous 240 cycletone which indicates that the end station 11 has been turned on and thata modulated carrier wave is being received from this station.

Similarly, when the end station 12 has been turned on by continuouslyradiating from the center station 10 a 1798.000 kilocycle carrier waveamplitude modulated with a 600 cycle signal in the manner previouslydescribed, the modulated wave radiated from the antenna 54 is receivedby the receiver 71 at the center station 10. The latter receiver detectsthe 600 cycle modulation component and passes it through the band passlter 92 and through section 78 of the oscillator selector switch 79 tothe signal monitor unit 90. Specifically, with both sections of theswitch 79 in their lower position, as illustrated in FIG. 3, the outputof the tilter 92 is applied across the gain control potentiometer 170and is passed through the amplifier and phase inverter unit 171 andthrough the push-pull amplitier 172 to the oscilloscope 173 where it iscompared with the highly stable 600 cycle signal developed by theoscillator 82. Obviously, the 600 cycle signal appearing at the outputof the push-pull amplifier 172 is also applied to the loudspeaker 174 toindicate that the end station 12 is on the air and that the receiver 71is receiving its amplitude modulated carrier wave.

In the event that the 1signal monitor 90 indicates that the endtransmitting station being monitored has failed to come on the air, itis only necessary -to remove the tone switching signal by opening theswitch 85 for a period of several seconds, for example, 10 seconds, andthen to reapply the modulation by again closing the switch. When thetone switch signal is removed for a 10 second period, the tone detectorrelay in the tone detector at the end station closes. This breaks thecircuit to the time delay relays 113 and 140 and, accordingly, when thetone switching signal is reapplied at the end of the 10 second interval,the relay 112 opens to apply a signal through the closed contacts ofswitch 137 to the time delay relay 140. Obviously, the relay 137 isclosed, in View of the fact that the end station 11 has failed to comeon the air. After the delay period produced by the relay 140, theratchet relay 122 is advanced to condition the transmitter transfercircuits so that at the completion of the delay cycle effected -by therelay 113 the operation will be automatically transferred to the standbytransmitter. The operator at the center station can then dispatch amaintenance crew to repair the inoperative transmitter so that both ofthe transmitters at the end stations will again be ready for operation.

In the event that it is desired to turn on both of the end stations 11and 12 to place the entire system in operation, the center station '10is rendered effective alternately to radiate its two carrier waves of1772.000 kilocycles and 1798.000 kilocycles and one of these waves ismodulated with a tone switching signal supplied by the oscillatorcontrol circuit 80. To eiiect this operation, the operator closes theswitches 98 and 99 and leaves the switches 96 and 97 open. If it isdesired to initiate operation by turning on the end transmitting station11, the oscillator selector switch 79 is positioned to supply a 240cycle signal through the amplifier 84 and through the closed switch 85to the modulator circuits of both of the transmitters 14 and 15. Thecommutator 21, as previousp ly described, is effective to render theoscillators of these transmitters alternately operative to develop thetwo carrier wave signals. The 240 cycle signal supplied from theamplifier control circuit is thus modulated upon both of the radiatedsignals. The 240 cycle signal modulated upon the 1798.000 kilocycle waveis reproduced by the receivers 58 and 59 at the end station 12 but isrejected by both of the filters 61 and 62 and, hence, has no effect onthe operation at this end station. The 240 cycle signal modulated uponthe 1772.000 kilocycle wave radiated from the cen-ter station isreproduced by the receivers 38 and 39 at the end station 11 and ispassed through the lilters 41 and 42 to the tone detector 43 in order toturn on the end station 1`1 in the manner described above. While the 240cycle signal appears at the output of the receivers 38 and 39 onlyduring the alternate intervals of operation of the center transmittingstation 10, as described above, the resistance-capacitance network at-the input circuit of the D.C. amplifier 110 in the tone detector 43holds over for this switching period and, hence, maintains the tonedetector relay 112 in its open position as long as the tone switchingsignal is being received. As soon as the transmitting station 11 isplaced on the air to radiate its 1798.600 kilocycle signal, thereceivers 58 and 59 at the end station 12 heterodyne the signalcontinuously radiated -from the end station 11 with the 1798.000kilocycle signal alternately radiated from the center station 10 todevelop a 600 cycle signal which is passed through the filters 61 and 62to operate the tone detector relay 63 in the manner indicated above,thereby placing the end transmitting station 12 on the air. When the endstation 12 is placed in operation the receivers 38 and 39 at the endstation 11 function to develop the 240 cycle beat frequency between thesignal continuously radiated from the end* station 12 and the 1772.000kilocycle signal alternately radiated from the center transmittingstation 10. Thus, the tone switching signal developed by the oscillator80 can be now removed by opening switch and the outputs of receivers 38and 39 at station 11 and of receivers 58 and 59 at station 12 willmaintain the end stations in operation. As previously indicated, in theevent of transmitter failure of a momentary nature at one of the endstations, the delay off relay 158 in the tone detector circuits at theend stations holds lthe transmitter of the other end station on the airso that if the failure corrects itself within the second delay periodContinous operation is maintained.

The system may also be turned on by supplying a 600 cycle tone switchingsignal from the oscillator control circuit 80 for modulation upon thecarrier waves alternately radiated from the center station. In thiscase, end station 12 is turned on first and end station 11 comes onthereafter in response to action of receivers 38 and 39 in heterodyningthe wave radiated from station 12 with the 1772.000 kilocyle waveradiated from center station l10.

With all the transmitting stations in operation the GREEN monitorreceiver '71 at the center station reproduces the 600 cycle referencesignal modulated upon the carrier wave radiated from the end station 12and passes the same through the band pass filter 92 to operate the -tonedetector 74. Similarly, the RED monitor receiver 72 reproduces the 240cycle reference signal modulated upon the carrier wave radiated from theend transmitting station 11 and passes this signal through the iilter 76to operate the -tone detector 75. Since the signals produced byreceivers 71 and 72 are used only for monitoring purposes and have noeffect on the position indications provided at the mobile receivingunit, it is 17 not necessary to employ standby receivers at the centerstation.

As illustrated in FIG. 4, the tone detectors 74 and 75 are similar tothe tone detector 43 previously described. However, in View of the factthat the input circuit of each of the tone detectors 74 and 75 isexcited by a single signal, a linear mixer need not be employed as theinput stage. Thus, the tone detector 74 includes an amplifier circuit180 which is excited by the 600 cycle signals from the band pass filter92. The output signals of amplifier 180 are passed through another 600cycle band pass filter 181 to a half-wave rectifier circuit 182 which issimilar to the rectifier 109 described above. Thus, the rectifier 182passes the negative half-cycles of the `240 cycle signal to the controlgrid of a D.C. amplifier 183 which is identical to the amplifier 110.The RC network in the input circuit of the D.C. amplifier 183 has a timeconstant which permits this circuit to hold and maintain the negativesignal during the normal switching period of the system and thus as longas signals are supplied to the tone detector 74, the tone detector relay184 remains open. The tone detector 75 is identical to the tone detector74 except that a 240 cycle band pass filter 185 is employed in place fthe filter 181 and, as a consequence, as long as the tone detector 75 isexcited by a 240 cycle input signal from band pass filter 76 the tone4detector relay 186 is maintained open.

The tone detector relays 184 and 186 are employed to control theoperation of the condition indicator 77 which provides appropriateindications in the event of failure of one or more of the endtransmitting stations. As long as `the transmitting system isfunctioning properly, the relays 184- and 186 remain in their openposition, illustrated in FIG. 4, and current from an A.C. source issupplied from terminal 192 through a manually operated reset switch 191through signal connector 190 and through the contacts of relays 184 and186 to one side of system-on signal light 196 and to one side ofoperating coil 194 of an alarm relay 195. The other sides of coil 194and of light 196 are connected together and to terminal 188 of thesource. Thus, the relay 195 is normally energized and light 196 isilluminated for normal system operation. However, in the event that the600 cycle signal input to the tone detector 74 disappears the relay 184will be closed to close contact 184a and complete a circuit forenergizing a signal light 187. The described circuit extends fromterminal 188 through a signal connector 189, through the signal light187, through the contact 184e, through connector 190 and through thereset switch 191 to the terminal 192. Thus, the absence of 600 cyclesignal input to the tone detector 74 is indicated immediately byillumination of thel GREEN signal light 187.

In the event that the 240 cycle signal input to the tone detector 75disappears, the relay 186 is closed to complete a circuit to a signallight 193. In the event that both of the tone -detectors 74 and 75 losetheir input signal, the circuit to the operating coil 194 of the alarmrelay 195 is opened and the circuit to the system-on light 196 is alsoopened. The relay 195 has two sets of contacts, one set 195o of whichfunctions, when the relay is deenergized, to lock the relay operatingcoil 194 across the terminals 188 and 192 and the other set 19512 ofwhich energizes an alarm bell to indicate system breakdown.

Failure at the end transmitting stations can be due to one of twocauses, either a loss of modulation or a loss of the carrier wavesignal. In the event that only modulation signal is lost from the endstation 12 and assuming that the rest of the system remains operative,the 600 cycle signal at the output of filter 92 disappears and the tonedetector relay 184 closes, but the tone detector relay 186 remains open.Under these conditions the GREEN indicating light 187 comes on, but thecircuit to the operating coil 194 is not broken, since this circuit isstill completed through the closed contacts of the relay 186. Therefore,the alarm bell does not sound. If, on the other hand, the end station 12loses its carrier wave signal, this action will soon be followed by aloss in beat signal output at the receivers 38 and 39 and, consequently,the end station 11 will go off the air at the end of the delay offperiod. Under these conditions signal output from both of the filters 76and 92 disappears and both of the relays 184 and 186 are closed to breakthe circuit to relay coil 194 and, hence, to sound the alarm bell.

Similarly, if the end station 11 should lose only modulation, signaldisappears from the output of filter 76 and the RED signal light 193becomes illuminated but the alarm kbell does not sound. Loss of carrierwave signal output -at the end station 11 is, of course, accompanied bya. shutdown at the end transmitting station 12 to operate the alarm bellof the condition indicator 77. Thus, if one of the lights 187 or 193 isilluminated without sounding the alarm the operator is informed that thestation identified by the particular light illuminated has lost itsmodulation. If the alarm bell rings the operator at the center stationcan surmise that one of the end stations Ihas lost its carrier and hemay determine the cause of the failure by interrogating the endstations. Interrogation of the end station 11 is performed as describedabove by rendering the center station effective by closing switch 96 toradiate its 1772.000 kilocycle signal and the 240 cycle signal outputfrom the amplifier control circuit 80 is modulated upon the wave. If theend station 11 is functioning properly, it will come on the air and itsoperation will be indicated by the signal monitor circuit in the mannerpreviously described. The interrogation of the end station 12 may, ofcourse, be effected by closing switch 97 to cause the center station toradiate its 1798.000 kilocycle signal and by modulating this signal withthe 600 cycle tone switching signal from the oscillator control circuit80. In this manner the source of the ytrouble can be determined and thenecessary steps can be taken to correct it. The alarm bell of thecondition indicator remains energized even though the standbytransmitter at the station which failed is rendered operative and thebell remains energized until the reset switch 191 is opened to break thecircuit to the relay 195.

As indicated above, the automatic transfer between the transmitters 14and 15 at the center station 10 is effected by the transfer unit 16 incooperation with the antenna changeover unit 23. The antenna changeoverunit 23 is generally similar to the unit 33 illustrated in FIG. 2 anddescribed above and includes means for applying a sample RF signal to an-RF detector circuit 198 (FIG. 5) in the unit 16 as, for example,through a signal connector 199. This RF signal is applied through aradio frequency filter 200 which may be tuned either to the 1772.000kilocycle frequency or to the 1798.000 kilocycle frequency. The selectedfrequency is passed through a half-wave rectifier circuit 201 having along time constant RC filter therein which rectifies the input signaland supplies negative half-cycles to the control grid of a D.C.amplifier 202. The long time constant RC filter, which may be either atthe output of the rectifier 201 or at the input of the D.C. amplifier202, maintains the negative signal for a period in excess of theswitching period by closing switch 96 to radiate its 1772.000 kilocyclesignal and the 240 cycle signal output from the amplifier controlcircuit 80 is modulated upon the wave. If the end station 11 isfunctioning properly, it will come on the air and its operation will beindicated by the signal monitor circuit 90 in the manner previouslydescribed. The interrogation of the end station 12 may, of course, be`effected by closing switch 97 to cause the center station to radiateits 1798.000 kilocycle signal and by modulating this signal with the 600cycle tone switching signal from the oscillator control circuit: 80. Inthis manner the source of the trouble can be determined and thenecessary steps can be taken to correct it. The alarm bell of thecondition indicator remains energized even though the standbytransmitter at the station which failed is rendered operative and thebell remains energized until the reset `switch 191 is opened to breakthe circuit to the relay 195.

As indicated above, the automatic transfer between the transmitters 14and 15 at the center station 10 is effected by the transfer unit andhiatus reducer 16 in cooperating with the -antenna changeover unit 23.The antenna changeover unit 23 is Igenerally similar to the unit 33illustrated in FIG. 2 and described above and includes means forapplying a sample RF signal to an RF detector circuit 198 (FIG. 5) inthe unit 16 as, for example, through a signal connector 199. This RFsignal is applied through a radio `frequency filter 200 which may betuned either to the 1772.000 kilocycle frequency or to the 1798.000kilocycle frequency. The selected frequency is passed through ahalf-wave rectifier circuit 201 having a long time constant RC filtertherein which rectifies the tinput signal and supplies negativehalf-cycles to the control grid of a D.C. amplifier 202. The long timeconstant RC filter, which may be either at the output of the rectifier201 or at the input of the D.C. amplifier 202, maintains the negativesignal for a period in excess of the switching period of the centertransmitting station and, as a result, a substantially continuousnegative signal is supplied to the amplifier 202 as long as the centerstation is radiating its radio frequency signals. The negative signalsupplied to the amplifier 202, of course, cuts off the plate circuitflow of this amplifier and opens the hiatus reducer relay 203i. In theevent of failure of the output signal supplied from the antennachangeover unit 23, the negative voltage supplied to the control grid ofthe D.C. amplifier 202 is removed and this amplifier starts to passplate current through the operating coil 204 of the relay 203, therebyto close the relay 203 and apply the Voltage [from a suitablealternating current source such as a 110 volt supply to a time delayrelay 205. The latter relay is indicated as being of the thermal typeand at the end of the predetermined time delay period the relay 205closes its contacts and completes a circuit to energize the operatingcoil 206 of a ratchet transfer relay 207. The ratchet relay 207 issimilar to the relay 122 described above in that energization of theoperating coil 206 actuates an operating pawl 208 to advance the ratchetrelay one step. Movement of the ratchet of the relay 207 drives a shaft209 to rotate switch operating cams 210 and 211 which function toalternately open and close switches 212 and 213 in an obvious manner.When switch 212 is closed, switch 213 is open, and supply voltage isdelivered through the operating coil of an antenna changeover relay 214to the transmitter 14. The antenna changeover relay when energizedconnects the output of the final amplifier of the transmitter 14 to theantenna circuit 24 and, when deenergized, connects the output oftransmitter 15 to the antenna circuit 24. When the ratchet relay 207 isso positioned that the switch 212 is opened, the vswitch 213 is closedand power is supplied through connector 215 to the transmitter unit 15.Under the latter conditions the antenna changeover relay 214 isdeenergized and the final amplifier of the transmitter 15 is connectedto the antenna circuit 24. A manually operated switch 216 may beemployed to bypass the ratchet relay 207 in order to place either thetransmitter 14 or the transmitter 15 in operation for purposes oftesting or calibration. Thus, when the switch 216 is in the upperposition illustrated in FIG. 5, power supply to the transmitter 14 isdelivered through the antenna changeover relay and through the closedcontacts of section 216a of switch 216. No power, of course, flows tothe transmitter 15v in view of the open circuit at switch section 216b.When the switch 216 is moved to its lower position, section 2166 of theswitch 216 functions to supply plate voltage to the electron dischargetubes of the transmitter 15 and section 216:1 interrupts the circuit totransmitter 14. The antenna changeover relay 214 is obviously controlledby the actuation of the switch 216 to effect the connection of theproper transmitter to the antenna circuit 24. In view of the foregoingdescription, it will be recognized that in the event that the signalinput to the unit 16 fails, the relay 203 is closed to operate the timedelay relay and after the predetermined delay period has expired theratchet relay 207 is advanced to automatically transfer the operation tothe standby transmitter. The delay period effected by the relay 205 isagain provided in order to prevent momentary failures from shutting downthe equipment, thereby permitting these momentary failures to correctthemselves without interfering with the operation of the system.

Turning now to the equipment comprising the failure indicator 70 which,as previously indicated, may be located either at the network oice 0r atany point remote from the transmitting stations 10, 11 and 12, thisequipment comprises a pair of receivers 220 and 221, respectively tunedto frequencies of 1772.120 and 1798.300 kilocycles. These receivers areexcited by a common loop receiving antenna 222 which is so oriented thatit accepts the signals radiated from all three of the stations 10, 11and 12. The GREEN receiver 220 is, of course, sufficiently selective toreject the signal radiated from the end station 11 and the 1798.000kilocycle signal alternately radiated from the center station 10, whilethe RED receiver 221 is sufiiciently selective to reject the signalcontinuously radiated from the end station 12 and the 1772.000 kilocyclesignal alternately radiated from the center station 10. The receiver 220heterodynes the signal from the end station 12 with the 1772.000kilocycle signal alternately radiated from the center station 10 todevelop a 240 cycle beat frequency which is passed through a band passfilter 223 to a GREEN heterodyne tone detector 224. The GREEN receiver220 also reproduces the 600 cycle reference signal alternately modulatedupon the carrier wave continuously radiated from the end station 12 andpasses this 600 cycle signal through a band pass filter 225 to a REDmodulation tone detector 226. The filter 225 is, of course, sufiicientlyselective to reject the 240 cycle beat frequency signal developed by thereceiver 220 and, correspondingly, the filter 223 rejects the 600 cyclereference signal reproduced by the receiver 220. The RED receiver 221heterodynes the signal continuously radiated from the end station 11with the 1798.000 kilocycle signal alternately radiated from the centerstation 10 to develop a 600 cycle beat frequency signal which is passedthrough filter 227 to a RED heterodyne tone detector circuit 228. TheRED receiver 221 also reproduces the 240 cycle reference signalalternately modulated upon the carrier wave continuously radiated fromthe end station 11 and applies this signal through a 2.40 cycle bandpass filter 229 to a GREEN modulation tone detector 230. The filter 229,of course, rejects the 600 cycle heterodyne signal developed by thereceiver 221 and the filter 227 rejects the 240 cycle reference signalreproduced by the receiver 221.

The tone detectors 224, 226, 228 and 230 are similar to the tonedetectors 74 and 75 illustrated in FIG. 4 and each functions to holdopen a tone detector relay as long as it is excited by input signals ofthe proper frequency. Of course, each of these tone detectors containsan audio frequency circuit tuned to the appropriate frequency to preventits operation by intermittent or spurious signals. The tone detectorrelay associated with the tone detector 224 is indicated at 224g, thetone detector relay associated with the tone detector 226 is indicatedat 226er, and so forth.

There are four conditions which can exist at the end stations to causeoperation of the system to break down, and these four conditions may beanalyzed and identified by reference to the four signals supplied to thefour tone detector circuits of the failure indicator 70. Failureindications respectively representative of the four conditions areprovided by employing the tone detector relay 21 to energize a relaymatrix indicated generally at 23-1 in the manner described below. Whenthe system is operating properly all four of the described tones aresupplied to the tone detectors and all of the relays are open. Endstation failure can occur in any one of the following four ways:

Condition 1.-'Ihe GREEN station carrier fails, in which event the GREENheterodyne signal disappears lfrom the output of the filter 223, theGREEN' reference signal is no longer developed by the receivers 38 and39 at the end station 11 and, accordingly, the GREEN reference signaldoes not appear at the output of the filter 229. The RED referencesignal no longer appears at the output of the lilter 225, since thecarrier wave for this signal is no longer available. At the end station11, development of the 240 cycle beat signal by the receivers 38 and 39terminates, but the RED station carrier remains on the air for 90seconds before the delay oif relay is operated. During this 90 secondperiod, the RED heterodyne signal is developed at the output of the bandpass lter 227 `and the tone detector 228 maintains its relay open while,as indicated above, the Irelays associated with the other three tonedetectors 224, 226 and 230 are all closed.

Condition 2,-The GREEN station modulation fails, in which case the REDreference signal appearing at the output of the band pass lilter 225disappears. Since all carriers are still present, both the GREEN and REDheterodyne signals and the GREEN reference signal are available and arepassed to their associated tone detector circuits. Under theseconditions the relay associated with the tone detector 226 is closed,while the relays associated with the other three detectors remain intheir open position.

Condition 3.-'1`he RED station carrier fails, in which event the GREENheterodyne signal is present for 90 seconds following the failure, whilethe RED heterodyne signal, the reference signal `and the GREEN referencesignal all disappear. Thus, under these conditions the relay associatedwith the tone detector 224 is open, while the relays associated with theother three tone detectors are closed.

Condition 4.--The RED station modulation fails, in which case the GREENreference signal disappears from the output of the band pass lter 229.The GREEN heterodyne signal, the GREEN reference signal and the REDreference signal are al1 present. Under these conditions the relay 230mlassociated with the tone detector 230 closes, while the relaysassociated with the other three tone detectors remain open.

As previously indicated, it is the function of the relay matrix 231 torespond to these on-otf conditions of the relays by indicating which ofthe four enumerated conditions is present in the event of end stationfailure. The indications are provided in the form of signal lights 232,233, 234 and 235, which respectively indicate the presence of conditionsl, 2, 3 and 4 enumerated above. The relay matrix 231 is excited from a110 volt supply appearing between line conductors 236 and 237. Thepresence of excitation voltage between these two conductors is indicatedby a power-on light 238. The excitation voltage for the D.C. powersupply and for the other circuits of the failure indicator 70 is derivedthrough an on-off switch 239 and through a signal connector 240. Lineconductor 237 is connected through a manually operated reset switch 241to one side of each of the signal lights 232, 233, 234 and 235, and toone side of a system-on light 242 which has its other side connectedthrough the normally closed contacts of a suitable time delay relay 243to the other line conductor 236. Thus, the system-on light 242 providesan indication that the contacts of relay 243 are closed and that therelay matrix 231 is conditioned for operation.

The line conductor 237 is also connected to the movable arm or pole ofeach of the tone detector relays 22451, 226er,

228:1 and 230a. The normally open contacts of the latter relays arerespectively connected to one side of the operating coils of relays 250,251, 252 and 253 in the matrix 231. The other sides of these relays areconnected together -to the movable arm of the time delay relay 243 andthrough the normally closed contact 243b of this relay to the lineconductor 236. Thus, as soon as one of the tone detector relays isclosed by the absence of signal at the input circuit of its associatedtone detector, a circuit is completed for energizing the associatedmatrix relay. Specifically, under condition l described above, relays250, 251 and 252 are simultaneously energized to close their contacts.Closing of contacts 250a and 251a connects the line conductor 236through connector 254 to the signal light 232 in order to indicate thatthe GREEN station carrier wave has failed. Connector 254 also functionsto connect the line conductor 236 through conductor 255 to parallelconnected operating coils of relays 256 and 257 in the matrix, the otherend of both of these relays being connected directly to line conductor237. Operation of relay 257 opens its contacts and prevents the light235 from being turned on. Light 234 cannot be turned on because it isconfronted by an open circuit at contact 253a of relay 253, while light233 is not illuminated because it is confronted by an open circuit atcontact 251b of relay 251,

Energization of the relay 256 breaks the normally closed contact 256]:and, hence, interrupts an energizing circuit for the operating coil ofan alarm relay 261. This energizing circuit extends from' line conductor236 through normally closed contact 25612, through normally closedcontact 258b, through normally closed contact 259b and through normallyclosed contact 260b. Deenergization of the operating coil of the relay261 causes the movable arms of this relay to occupy the positionillustrated in FIG. 6 so that contact 261e is closed to complete acircuit to an alarm not shown. The alarm, of course, apprises theoperator at the failure indicator station that normal operatingconditions no longer prevail. Closing of contact 26119 of relay 261connects the line conductor 237 across the 4operating element 243a ofthe time delay relay 243. If the cause of the operational failure of theGREEN carrier does not remedy itself within the delay period of therelay 243, this relay becomes energized to break contact 243b and,hence, to remove operating p0- tential from the relays 250, 251, 252 and253. If, on the other hand, the cause of the failure is remedied beforethe cycle of relay 243 has been completed, the tone detector relays allrevert to their open conditions and the relay 261 is again energized tobreak the circuit to time delay relay 243 and to shut olf the alarm. Thetime delay provided by relay 243 is less than that provided by thetransfer delay relay employed at the end stations.

Operation of the relay 256 has another important result, in that itcloses contact 256a and completes a circuit to the operating coil ofrelay 256 from conductor 254 through connector 255 in order to lock thisoperating coil across the voltage supply line. Cessation of current owin the relays 250, 251, 252 and 253 insures that further events takingplace at the tone detectors have no elfect, due to the fact that thecontacts of the time delay relay 243 are open when the relay 261 isdeenergized. Thus, even though the standby transmitter at the endstation which failed is subsequently rendered effective, this can haveno elfect on the failure indicator 70. Specifically, the contacts of thetime delay relay 243 are maintained in their open position, due to thefact that the operating element 243er is locked directly `across theline conductors 236 and 237. The failure indicator also functions tomaintain the signal light 232 illuminated in view of the fact that thecoil of relay 256 is locked across the line and the now closed contact256:1 is effective to complete the circuit from line conductor 236through conductor 254 to the light 232. Thus, even though the operatormay be temporarily absent when the failure occurs, the failure 23indicator maintains its indications to inform him that repairs must bemade at the identified end station.

To reset the unit for operation after the failure has been corrected,either by way of corrective repairs or otherwise, it is necessary merelyto open the manually operable reset switch 241 which breaks the circuitto all of the signal lights and, at the same time, interrupts thecircuit to the operating element 243a of the time delay relay 243.Operation of the reset switch also breaks the energizing circuit for therelays 256 and 257 and allows the contacts of these relays to return tothe position illustrated in FIG. 6. Thus, when the reset switch 241 isagain closed, all of the circuit elements of the failure indicator 70again occupy the positions illustrated in FIG. 6.

If the second condition prevails, relay 252 is energized to complete acircuit from line conductor 236 through its closed contact 252k, throughthe normally closed contact 251b, through signal connector 262 andthrough the normally closed contact 263@ of relay 263 to the GREENmodulation signal light 233. Signal light 232 remains off, since it isconfronted by an open circuit at contact 250e, signal light 235 remainsoff because its energizing circuit is open at contact 250b and signallight 234 remains olf because it is confronted by an open circuit atcontact 253:1. The energized conductor 262 is connected to one end ofthe operating coil of relay 25S so that the latter relay is actuated toopen its contacts in order to lock the operating coil across the line byclosing of contacts 258:1 and to interrupt the energizing circuit forthe operating coil of relay 261 by breaking contact 258b. Deenergizationof the operating coil of relay 261, of course, actuates the alarmcircuit and, at the same time, applies an operating voltage across theoperating element 243m of the time delay relay 243 With the resultsdescribed above.

Under condition 3 described above, relays 250, 252 and 253 are allactuated, While relay 251 remains in its deenergized state. Energizationof the three identified relays completes a circuit from the lineconductor 236 through the now closed contact 253er, through the nowclosed contact 252a and through signal connector 265 to illuminate theRED carrier signal light 234. The energized conductor 265 appliesoperating voltage across the operating coils of relays 259 and 263.Actuation of the relay 263 opens its contact 263er and prevents thesignal light 233 from being illuminated. The energizing circuit forsignal light 235 is open at contact 253/J and, hence, light 235 remainsoff. The circuit to signal light 232 is open at contact 251er and, as aresult, this light does not become illuminated. Operation of the relay259 breaks the contact 259b to interrupt the circuit to the alarm relay261 and, at the same time, closes contact 259@ to lock the operatingcoil 259 across the line, with the results described above.

Under the fourth condition, the relay 250 is energized, while the relays251, 252 and 253 remain in their deenergized conditions. When relay 250is actuated, a circuit is completed from line conductor 236 throughnormally closed contact 253b, through the now closed contact 25011,through conductor 270, through the closed contacts of relay 257 andthrough conductor 271 to the signal light 235. The light 235 is thusilluminated to indicate that the RED station modulation has failed. Thesignal light 232 is not illuminated, in view of the fact that `itsenergizing circuit is opened iat contact 251a while the circuit forlight 234 is open at contact 252e and the energizing circuit for light233 is open at contact 252i).

Application of energizing potential to the conductor 270 causes anoperating current to be passed through the coil of relay 2160 in orderto actuate the contacts of the latter relay. Opening of contact 260b, ofcourse, breaks the energizing circuit for the relay 261 with the resultsdescribed above while closing of contact 266er locks the coil of relay260 across the line, in view of 'the fact that the contact 26011 isconnected directly to the line conductor 236.

In view of the foregoing description, it will be recognized that thefailure indicator 71B functions to provide `an immediate indication ofoperating failure occurring at either of the end stations 11 or 12. Thelock-on feature of the relay matrix 231 maintains the indication untilthe trouble has been corrected and until the reset switch 241 is openedmanually.

In view of the detailed description above, it will be recognized that-the system illustrated in FIGS. l through 6 may be turned on or offfrom the center transmitting station 10 and, accordingly, the presentinvention avoids the use of a continuous station watch at the endtransmitting stations 11 and 12. Moreover, all three of the transmittingstations include standby transmitting equipment which is automaticallyrendered operative in the event of failure of the main transmittingequipment. Furthermore, the system includes means at the center -stationand at the failure indicator 70 for monitoring the system in suchfashion that operational failure is immediately indicated and also foridentifying and locating the source of the trouble.

The invention may also be employed in a transmitting system of the typedisclosed and claimed in U.S. Patent No. 2,513,316 to lames E. Hawkins,assigned to the same assignee as the present invention. Such a system isillustrated in FIGS. 7 and 8, wherein there is shown a centertransmitting -station 310 and a pair of spaced apart end transmittingstations 311 and 312, which are adapted to radiate signals of differentfrequency separated from each other by a small audio frequencydifference. The end stations 311 and 312 are preferably locatedapproximately equal distances from the center station 310 and are sodisposed that an imaginary base line extending between the centerstation 310 and the end station 311 is angularly related to a similarbase line extending between the center station 310 and the end station312. The transmitting system also includes a reference station 313located at some distance from the three transmitting stations 310, 311and 312 and effective to transmit reference signals in the form ofmodulation signals developed from heterodyning waves received from thestations 310, 311 and 312.

Equipment is employed for turning on and off three of the stations 31),311 and 312, under the control of modulated signals radiated from thereference station 313. In addition, each of the four transmittingstations includes standby transmitting equipment which is automaticallyrendered effective in the event of failure of the main transmittingfacilities.

Furthermore, monitoring equipment is provided at the reference station313 ifor indicating failure of one of the transmitting stations and foridentifying the station at which lthe failure occurred. This indicationis maintained even after the standby transmitting equipment has beenrendered operative, so that the necessary repairs can be made withoutrequiring the system to be shut down.

The equipment provided at the center station 310 to effect the aboveenumerated results comprises a first transmitter 314 and a secondtransmitter 315, `only one of which is adapted to be rendered operativeat a time, so that the inoperative transmitter serves as a standby unit.Each of these transmitters includes an oscillator or signal generator, afinal amplifier and one o-r more intermediate amplifiers and each isadapted to develop a signal having a frequency of 1798.240 kilocycles.The output signal from the final amplifier of both of the transmitters314 and 315 is supplied through an antenna changeover unit 323 to anemitting antenna circuit 324. The antenna changeover unit 323 isidentical to similar units described above and functions automaticallyto connect the antenna circuit 324 to the output of the propertransmitter 314 or 315 in the manner previously de- 25 scribed. Theantenna changeover unit 323 also supplies a sample of the RF outputsignal supplied to the antenna circuit 324 through a signal connector325 to a transfer unit and RF detector 316. The latter unit is similarto the `transfer unit illustrated in FIG. 2 and employed at the endstation of the transmitting system illustrated in FIG. 1. However, thetransfer unit 316 includes Ian RF detector Ilike the detector 198illustrated in FIG. 5 in place of the modulation detector 138. This RFdetector, of course, functions to control the operation of a relay tomaintain the relay open as long as an RF signal is being `supplied tothe antenna 324 and to close the relay in the event of signal failure.This relay corresponds to the relay 137 of the transfer unit shown inFIG. 2. In all other respects the transfer unit 316 is identical to theunit 32 described in detail above.

The remote control at center station 310 is effected through control ofthe transfer unit 316 by means of a tone detector 317 which is identicalto the tone detector 74 illustrated in FIG. 4. The tone detector 317 isexci-ted by the signals developed by a co-ntinuously operated receiver318 tuned to a ifrequency of 1772.000 kilocycles land sufficientlyselective to reject signals from the end stations 311 and 312. Thesignal input terminals of the receiver 318 are excited by the :signalspicked up by a loop receiving antenna 320 which is so oriented that thesignals emitted from the emitting antenna 324 are nulled, While thesignals radiated from lthe reference station 313 are accepted. Thesignals developed by the receiver 31S are passed through filters 321,322 and 326 respectively tuned to frequencies of 240 cycles, 360 cyclesand 600 cycles to a non-linear mixer circuit 327. The output signals ofthe latter mixer circuit are passed through a band pass filter 328 tunedto a frequency of 600i cycles to the tone detector 317. Standby oralternate receiving equipment may, of course, be employed if desired,but this is not illustrated in the drawings'.

The equipment provided at the end station 311 comprises a pair oftransmitters 330 and 331, each developing a signal having a frequency of1798.000 kilocycles. Only one of these transmitters is employed at atime, the other unit serving as a standby transmitter in case ofoperational failure of the main transmitting equipment. Each transmitterincludes an oscillator or signal generator and one or more amplifyingstages, the final amplifying stage of each transmitting unit beingconnected to deliver its output signals through `an antenna changeoverunit 333 Ito `an .antenna circuit 334. The antenna changeover unit 333is like the unit 33 employed at the end station 11 in the systemillustrated in FIG. 1 and includes means for automatically applying theoutput of the proper transmitter 330 or 331 to the antenna circuit 334.Transmitters 330 and 331 are rendered effective automatically by meansof `a transfer unit and RF detector 332 which is excited by a samplesignal supplied from the antenna changeover unit 333 via a signalconnector 335. The unit 332 is identical to the unit 316 employed at theend station and is controlled by a tone detector 336 which corresponds.to the detector 317 at the center station. The tone detector 336 isexcited by the signals developed by `a receiver 338 land passed througha 240 cycle band pass filter 339. The input tenninals of the receiver338 are excited by the signals picked up by a loop receiving antenna 340ywhich is so oriented that a maximum response is provided for signalsarriving from the reference station 313, while the high power signalsemitted from the antenna 334 are effectively nulled or eliminated. Thereceiver 338 is centered tuned to a frequency of 1772.000 kilocyclesand, hence, is adapted to reject the signals radiated :from both of thestations 310 and 312, while accepting the signals radiated from thereference station 313.

The equipment provided at the end station 312 comprises a pair oftransmitters 350 and 351 for developing signals having a frequency of1798.600 kilocycles, which are passed through an antenna changeover unit353 to a radiating antenna circuit 354. Only one of the transmitters 350or 351 is effective at any particular time, the remaining unit being astandby transmitter which is rendered operative automatically in theevent of failure of the effective transmitter under the control of atransfer unit and RF detector 352. The transfer unit 352 is identical tothe unit 316 employed at the center station and is excited by RF signalspassed from the antenna changeover unit 353 through a signal connector355 to the RF detector portion of the unit 352. Remote control of theend station 312 is effected by controlling the operation of the transferunit 352 through a tone detector 356 similar to the tone detector 317referred to above. The tone detector 356 receives excitation from theoutput signals of a receiver 358, which signals are passed through aband pass filter 359 tuned to a frequency of 360 cycles. The signalinput terminals of the receiver 358 are excited by the signals picked upby `a loop receiving antenna 360 which is so oriented that the signalsemitted from the antenna 354 are nulled or eliminated. The receiver 358is center tuned to a frequency of 1772.000 kilocycles and its inputstages are sufficiently selective to reject the signals arriving at theantenna 360 from the center station 310 and from the end station 311,While passing the signals arriving from the reference station 313.

Turning now to the equipment provided at the reference station, thisequipment includes a pair of transmitters or signal generators 370 and371, each of which includes an oscillator developing signals having afrequency of 1772.000 kilocycles, an amplitude modulation circuit .and afinal amplifier stage. These transmitters are, of course, renderedoperative one at a time and the inoperative transmitter functions as astandby unit. The final amplifier stage of the operative transmitter isautomatically connected to an antenna circuit 373 by means of an antennachangeover unit 372 like the unit 323 described above. Automatictransfer -to the standby transmitter in the event of failure of theoperative transmitter is effected by a transfer unit and modulationdetector 374. The unit 374 is similar to the transfer unit illustratedin FIG. 5, but it includes ya modulation detector circuit like thecircuit 138 shown in FIG. 2 in place of the RF detector circuit 198. Themodulation detector circuit is excited by signals supplied from theantenna changeover unit 372 and, in the absence of such signals, thedetector relay at the output of the modulation detector circuitfunctions to effect the automatic transfer to the standby transmitter inthe manner previously described.

To provide reference signals for phase comparison at the mobilereceiving unit with heterodyne signals derived from waves radiated fromstations 310, 311 and 312 in conventional manner, the reference station313 includes a reference receiver 375 which is center tuned to afrequency of 1798.300 kilocycles and which accepts the signals radiatedfrom all of the stations 310, 311 and 312. To this end, the signal inputterminals of the receiver 375 are excited by signals picked up by a loopreceiving antenna which is so positioned that the signals radiated fromthe antenna 373 are nulled. The receiver 375 functions to heterodyne thethree accepted signals and develops the beat frequencies therebetween.The 240 cycle beat frequency signal developed as a result ofheterodyning the wave received from the end station 311 with the wavereceived from the center station 310 is passed through a 240 cycle bandpass filter 377 to the modulation circuits of both of the transmitters370 and 371. Similarly, the 360 cycle beat frequency signal developed asa result of heterodyning the signal radiated by the center station 310with that radiated by the end station 312 is passed through a 360 cycleband pass filter 378 to the modulator circuits of the transmitters 370and 371. The filters 377 and 378 both reject the 600 cycle beatfrequency signal developed as a result of heterodyning the two wavesreceived from the end stations 311

1. IN A TRANSMITTING SYSTEM OF THE HYPERBOLIC TYPE EMPLOYING RADIATION OF WAVES FROM A PLURALITY OF SPACED APART POINTS, A TRANSMITTING STATION AT ONE OF SAID POINTS COMPRISING A FIRST TRANSMITTER AND A SECOND TRANSMITTER, A CIRCUIT FOR RENDERING SAID TRANSMITTERS EFFECTIVE ONE AT A TIME SO THAT THE INEFFECTIVE TRANSMITTER SERVES AS A STANDBY TRANSMITTER, A WAVE SIGNAL RECEIVER RESPONSIVE TO A WAVE RADIATED FROM A POINT REMOTE FROM SAID STATION, AND MEANS RESPONSIVE TO SIGNALS FROM SAID RECEIVER FOR CONTROLLING SAID CIRCUIT TO TRANSFER THE OPERATION FROM ONE OF SAID TRANSMITTERS TO THE OTHER IN RESPONSE TO THE INITIAL APPLICATION OF SIGNALS FROM SAID RECEIVER. 